1. Field of the Invention
The invention relates to fabrication of graded-index plastic optical fiber.
2. Discussion of the Related Art
Glass optical fiber has become a significant transmission medium in recent years, particularly for long distance transmission applications. Such optical fiber has not found significant usage, however, in smaller scale applications, such as distribution of fiber to the desk in local area networks. In particular, glass optical fiber has not been as cost effective as, for example, copper wire, and connectorization of glass fiber (which needs well-polished end faces) requires substantial time, skilled personnel, and precise connector components. There has been interest, therefore, in pursuing plastic optical fiber (POF). POF offers many of the benefits of glass optical fiber, but is expected to be more cost effective, and POF also offers a larger core that makes connection easier.
Initially, step index POF (having a core of one refractive index, surrounded by a cladding of a lower refractive index) was manufactured and used. Unfortunately, the modes propagating in a step index fiber experience an undesirably high level of dispersion, thereby limiting the fiber""s capability. In response to this problem, graded index POF (GI-POF) was developed, which possesses a varying refractive index from the core to the cladding layer. GI-POF exhibits a lower level of mode dispersion, thereby providing improved properties. GI-POF, however, was more difficult, and thus more expensive, to manufacture than step index POF. Improved methods for manufacturing GI-POF were therefore sought.
One method of forming GI-POF is to start with a preform, similar to the preform from which glass optical fiber is generally drawn. See, e.g., U.S. Pat. Nos. 5,639,512 and 5,614,253, which discuss a process for chemical vapor deposition (CVD) formation of a preform for GI-POF. According to the process, a polymer and a refractive index modifier are deposited onto a rod, and the amount of refractive index modifier is varied during the deposition to provide the desired refractive index profile. While such preforms are useful for preparing GI-POF, easier processes are desired.
One alternative to preform-formation is extrusion, which is commonly used with plastics to form a variety of items. Extrusion was expected to be quicker and cheaper than forming and drawing a preform, but the need for a graded refractive index profile created complications. U.S. Pat. No. 5,593,621 (the ""621 patent) discusses an extrusion process for GI-POF. According to the ""621 patent, GI-POF is manufactured by extruding one material circumferentially around another material, e.g., by use of a concentric nozzle. At least one of the materials contains a diffusible material having a distinct refractive index, such that the diffusion of the material provides the desired refractive index contrast. The method of the ""621 patent appears to offer a functional process, but also appears to exhibit several drawbacks.
In particular, it is not clear that the process is able to be performed without providing a delay time (stopping the flow of material) or a very slow extrusion speed, to allow the diffusible material sufficient time to diffuse. Specifically, the examples disclose a small distance, 3 cm, between the outlet of concentric nozzle 5 (see FIG. 1) and the outlet of core nozzle 3. Thus, the two materials are in contact only over this small distance before exiting the apparatus. It is unclear whether this small contact distance allows sufficient diffusion, without requiring either intermittent stoppage or an extremely slow extrusion speed. It appears that either stoppage or low speed was used, because, for example, Embodiment 6 states that diffusion was effected for about 3 minutes within this contact region, and Embodiments 7, 8, and 9 all state that diffusion occurred for about 10 minutes in the contact region. Unfortunately, the reference does not disclose an extrusion speed nor make clear whether the process had to be halted intermittently. In addition, there is no information on how to predict the refractive index profile in the resulting fiber, and trial-and-error is apparently required to find appropriate process parameters.
An improved extrusion technique for plastic optical fiber is reflected in co-assigned U.S. patent application Ser. No. 09/321,050 filed May 27, 1999 (our reference Blyler 43-18-1-18), the disclosure of which is hereby incorporated by reference. Further improvements in such extrusion methods are desired.
The invention provides an improved process for extruding plastic optical fiber without the need to prepare a preform. Specifically, it was discovered that conventional extrusion techniques, e.g., screw extruders, tended to introduce an undesirable amount of particulate contaminants which increased the loss of the drawn fiber. To overcome this problem, the invention substantially reduces the number of mechanical interactions that contribute to such contamination. The process of the invention does so by using fluid (typically gas) pressure, instead of screw extruders, to induce polymer flow. The process also controls the flow characteristics of the polymer, or halts the flow altogether, without mechanical controls. Specifically, the temperature of the sections through which the polymer flows is controlled, such that it is possible to bring the polymer to a desired flow rate, or even to a solid state to provide a plug. Using the process of the invention, high quality graded index plastic optical fiber is possible, e.g., GI-POF that exhibits a relatively low loss of 50 dB/km or better.